Polycomb group (PcG) and trithorax group (trxG) proteins are known to be part of the cellular memory system (Francis and Kingston (2001) Nat. Rev. Mol. Cell Biol. 2:409-421; Simon and Tamkun (2002) Curr. Opin. Genet. Dev. 12:210-218). Both groups of proteins are involved in maintaining the spatial patterns of homeotic box (Hox) gene expression, which are established early in embryonic development by transiently expressed segmentation genes. In general, PcG proteins are transcriptional repressors that maintain the “off state,” and trxG proteins are transcriptional activators that maintain the “on state.” As members of PcG and trxG proteins contain intrinsic histone methyltransferase (HMTase) activity, PcG and trxG proteins may participate in cellular memory through methylation of core histones (Beisel, et al. (2002) Nature 419:857-862; Cao, et al. (2002) Science 298:1039-1043; Czermin, et al. (2002) Cell 111:185-196; Kuzmichev, et al. (2002) Genes Dev. 16:2893-2905; Milne, et al. (2002) Mol. Cell 10:1107-1117; Muller, et al. (2002) Cell 111:197-208; Nakamura, et al. (2002) Mol. Cell 10:1119-1128).
Biochemical and genetic studies have provided evidence that Drosophila PcG proteins function in at least two distinct protein complexes, the Polycomb repressive complex 1 (PRC1) and the ESC-E(Z) complex, although the compositions of the complexes may be dynamic (Otte and Kwaks (2003) Curr. Opin. Genet. Dev. 13:448-454). Studies in Drosophila (Czermin, et al. (2002) Cell 111:185-196; Muller, et al. (2002) Cell 111:197-208) and mammalian cells (Cao, et al. (2002) Science 298:1039-1043; Kuzmichev, et al. (2002) Genes Dev. 16:2893-2905) have demonstrated that the ESC-E(Z)/EED-EZH2 complexes have intrinsic histone methyltransferase activity. Although the compositions of the complexes isolated by different groups are slightly different, they generally contain EED, EZH2, SUZ12, and RbAp48 or Drosophila homologs thereof.
In addition to Hox gene silencing, EED-EZH2-mediated histone H3-K27 methylation has been shown to participate in X-inactivation (Plath, et al. (2003) Science 300:131-135; Silva, et al. (2003) Dev. Cell 4:481495). Recruitment of the EED-EZH2 complex to Xi and subsequent trimethylation on histone H3-K27 occurs during the initiation stage of X-inactivation and is dependent on Xist RNA. Furthermore, EZH2 and its associated histone H3-K27 methyltransferase activity was found to differentially mark the pluripotent epiblast cells and the differentiated trophectoderm (Erhardt, et al. (2003) Development 130:4235-4248). Consistent with a role of EZH2 in maintaining the epigenetic modification patterns of pluripotent epiblast cells, Cre-mediated deletion of EZH2 results in loss of histone H3-K27 methylation in these cells (Erhardt, et al. (2003) Development 130:4235-4248). Further, studies in prostate and breast cancer cell lines and tissues have revealed a strong correlation between the levels of EZH2 and SUZ12 and the invasiveness of these cancers (Bracken, et al. (2003) EMBO J. 22:5323-5335; Kirmizis, et al. (2003) Mol. Cancer Ther. 2:113-121; Kleer, et al. (2003) Proc. Natl. Acad. Sci. USA 100:11606-11611; Varambally, et al. (2002) Nature 419: 624-9), indicating that dysfunction of the EED-EZH2 complex may contribute to cancer.
Given that the EED-EZH2 complex-mediated histone H3-K27 methylation participates in a variety of important processes, there is a need in the art for agents that modulate the activity of this complex and methods for identifying the same.